Jet engines employed in modern aircraft require careful control over operational characteristics for optimum performance. Multiengine craft also require a balancing of engine performance so that, for example, the engines operate similarly when at a common throttle setting. This calibration of engine parameters, dubbed "trimming" by those skilled in the art, occurs following manufacture of the aircraft and then periodically over its service life, usually about every 200 operating hours.
Typically, the aircraft engines of interest are dual spool engines having non-mechanically linked low pressure compressor and high pressure compressor stages. Variable stator vanes in the compressors modulate combustion air in response to the flight regime of the craft, with the high pressure compressor usually being of greater interest in this regard; coordinating air availability with fuel for that purpose. A fairly complex hydromechanical linkage provides the requisite level of control. Engine speed must also be coordinated with these same thoughts in mind. Components in this overall assembly responsible for fuel control, both directly and indirectly, are those which must be trimmed to balance properly the diverse engine conditions existing throughout flight. The parameters requiring trim vary with regard to engine model and manufacturer; but, in virtually all cases, one or more of the following engine parameters require periodic adjustment for optimal engine operation: (i) engine vane and bleed control, (ii) intercompressor airflow bleed (sometimes referred to as "station 3.0 bleed"), (iii) approach idle, (iv) minimum idle, (v) takeoff power.
A number of different approaches have been adopted over the years to trim these aircraft engine parameters. One of the earliest techniques required quite a number of ground personnel, some of whom would work outside the craft and at least one of whom would be placed in the cockpit in order to inspect gauges and the like for monitoring pertinent engine conditions. Insofar as cockpit instrumentation typically does not include gauges which are dedicated to each of the required engine parameters of interest, ancillary equipment would be needed at the engine location. Readings would be taken with the engine or engines at a predetermined power level; manufacturer's charts would be consulted to determine the type and extent of necessary correction; the engine would be shut down; and the correction would then be made on an ad hoc basis. That procedure would be followed repetitively until each parameter had been successfully trimmed through this laborious technique. The approach was a very tedious one, insofar as the actual adjustment of a trim screw borne upon the aircraft engine required the engine to be shut down, the cowling opened and the technician to estimate how far the trim screw ought to be turned to achieve the desired result; that followed by repositioning of the cowling, restarting the engine, and rereading the instrumentation. To a certain extent manufacturer's specifications assisted this technique, inasmuch as each trim screw was associated with a collar or the like having indentations disposed equiangularly thereabout in order that a complete revolution of the screw was divided into a number of "clicks". Manufacturer'specifications were geared to movement of the trim screw over a predetermined number of "clicks" as determined by the indentations to achieve a desired amount of adjustment. Still, the procedure was highly labor intensive, consuming many man-hours of time to achieve proper trim of an engine.
Later, this ad hoc approach gave way to a slight improvement which eliminated at least the need to open and close the engine cowling repetitively. Flexible drive members associated with a trim head were removably installed on the engine in advance of a trimming operation. The flexible drives were disposed to cooperate with a respective engine trim screw to turn the same, terminating at a location outside the cowling allowing an operator to make adjustments from that exterior position. In a related approach, a gear box disposed externally of the cowling provided electromechanical linkage between the flex-drive cables and an operator in the cockpit. However, even these approaches, while improvements, suffered problems nonetheless. Most notably, the hysteresis of flexible drives made their use awkward and sometimes unreliable (i.e., due to windup and backlash associated with several feet of flex-drive cables), regardless of direct or indirect manipulation at or near the engine.
No truly efficient and virtually foolproof method of effectuating remote engine trim has yet to be devised. However, certain patented systems have been suggested in the past for various types of other engine control and warrant brief mention for background purposes.
U.S. Pat. Nos. 4,158,884 and 4,116,052 are generally representative of engine trim test sets; each providing means to examine into the engine parameters requiring trim adjustment but not with the collateral ability to undertake necessary trim. Viewing the '884 reference as representative, the device therein disclosed is a portable test set for taking field measurements of the engine conditions of interest and inputting data representative thereof to a microprocessor. The microprocessor uses these input signals, along with stored data, to calculate values for measured trim parameters and outputs that result to a digital display for operator observation.
Of some incidental interest along conceptually related lines, U.S. Pat. No. 4,056,732 discloses an airborne electronic engine control system. This is flight, as opposed to ground, equipment designed to regulate the AC electrical power provided by an engine driven alternator, all with an eye toward controlling the performance of a gas turbine engine. But, the ability to achieve remote trim remains beyond the intended scope of that device.
Considering the fact that the present engine trim control unit employs remotely actuable drive members for effectuating trim commands, other patented systems come to the fore in terms of background. Among these are included U.S. Pat. Nos. 3,813,063, 3,839,860, and 3,852,956. Each of these references concerns an automatic aircraft engine pressure ratio ("EPR") control system. These systems deal with EPR equalization for gas turbine engines and/or automatic thrust management which are permanent parts of the engine installation in the airplane and are used in day-to-day flight operation. These types of trim systems are quite different from the engine trim control units employed to monitor an engine, either at the time of initial aircraft fabrication or as part of a routine, periodic maintenance schedule, and thence to trim engine parameters; whereupon the trim unit and ancillary equipment are stripped from the craft and its engine.